This invention relates to providing a novel micro-machined structure and a method for providing the structure.
Micro-machining of silicon (and other materials) to provide structures which have dimensions of the order of micro-meters is known. This technique is used to provide structures fabricated from bulk material and shares some common features with the techniques used to fabricate integrated circuits. For instance it is disclosed in WO 96/10630 to provide tetrahedral structures, or barbs, which have a base dimension of between 30 micro-meters to 80 micro-meters.
It is disclosed in U.S. Pat. No. 5,591,139 to fabricate a microneedle parallel to the surface of a substrate wherein the walls of the needle are fabricated from SiO2 deposited by low pressure chemical vapour deposition. To form such a needle a large number of steps are required.
U.S. Pat. No. 5,262,128 discloses micro-projections having ducts therethrough. It is claimed that the micro-projections may be fabricated by the Liga process. The skilled person will appreciate that this process is not a technique for microstructuring silicon and is used for processing metals and polymeric materials. Further, all structures fabricated by the Liga process must have edges which are substantially perpendicular to the surface of the substrate from which they are formed. From the remainder of this document it will be apparent that the Liga process is not suitable for fabricating the micro-projection disclosed herein.
Further, U.S. Pat. No. 5,262,128 states that the structures disclosed therein were fabricated by techniques well-known in the integrated circuit industry. It is disputed that in 1989, the application date of U.S. Pat. No. 5,262,128, silicon processing techniques, were known which could produce the structures disclosed therein. It is even disputed that such techniques were known in 1993, the publication date of U.S. Pat. No. 5,262,128. Techniques for fabricating high aspect ratio ducts have only recently become known. This is no discussion in U.S. Pat. No. 5,262,128 of how to form the structures shown therein, thus the teachings are not enabling.
An alleged Internet disclosure purportedly dated 22 Jun. 1998 by Georgia Tech (www.gtri/gatech/edu/res-news/needles.htnl) discloses arrays of micro-projections. On the second page of this document it is disclosed that future work includes attempting to fabricate needles which are hollow by some unspecified process. Therefore, the purported Georgia Tech document does not provide an enabling disclosure of hollow micro-projections (since they cannot make them since this is indicated as future work).
Further, the skilled person will appreciate that the fact that other research institutions other than the applicants are working on providing the structures shown in U.S. Pat. No. 5,262,128 points at the non-enabling nature of U.S. Pat. No. 5,262,128.
According to a first aspect of the invention there is provided a method of providing a micro-projection on the surface of a first material, the micro-projection having a base portion adjacent the first material, and a remote, or tip, portion, and a duct at least in a region of the tip portion and the method comprising micro-machining the first material to provide the micro-projection and duct.
The method may comprise forming the duct such that it passes between the tip portion and the base portion.
Indeed, in some embodiments the duct may be formed such that it passes entirely through the micro-projection and may be through the first material supporting the micro-projection. Such a method provides a micro-projection onto the surface of a material through which matter may be passed, possibly a fluid.
The first material may be a semi-conductor, such as silicon material, or other material with a crystal lattice structure. The first material may be glass.
Preferably the base portion has a width of less than approximately 1000 xcexcm and most preferably less than approximately 100 xcexcm and possibly below 10 xcexcm.
The tip portion of the micro-projection may have an apex. The method may fabricate the micro-projection such that the duct is coincident with the apex of the micro-projection. This may provide a needle like structure which has a number of uses.
In one embodiment the method may comprise fabricating the duct in a piece of material and subsequently forming the micro-projection around the duct. If such a method is used the method may comprise fabricating the duct and then fashioning the material such that the tip portion of the micro-projection coincides with the duct. Indeed, the apex of the micro-projection may be fabricated to be coincident with the duct.
The duct may be formed by a number of processes and any one of the following list may be suitable: deep dry etching, laser ablation, light assisted anodisation of n-type silicon in HF acid, focused ion beam milling. Each of these techniques may provide a duct of suitable dimensions for the realisation of the method, and will be known to the person skilled in the art.
A mask layer may be formed on to the surface of the first material. The mask layer may comprise silicon dioxide should the first material be a bulk silicon crystal.
An aperture may be formed into the mask layer using lithographic and etching techniques as will be known to the person skilled in the art.
The first material may be removed from a region underneath the aperture in the mask using an etching technique. This step provides one way of forming the duct.
The duct, or a hole that will become the duct, may be formed in a sheet of planar material. A plurality (e.g. of the order of hundreds of thousands) of ducts may be formed simultaneously.
Once a suitable duct has been formed in, for example, a sheet of material, the micro-projection may be formed from the material by a number of techniques. For example any one of the following list may be suitable: anisotropic wet etching of silicon using liquid alkaline etches, focused ion beam milling, or transferring the pattern to the silicon from a domed resist mask using some form of plasma/ion beam etching.
The mask layer may be removed from the first material. A second masking layer may be created on to the first material which may be silicon dioxide. The second masking layer may be grown (perhaps by a low temperature oxide process LTO) or may be deposited (perhaps by plasma enhanced chemical vapour deposition PECVD). An advantage of removing the first masking layer and creating a second masking layer is that the inside surface of the duct is covered and protected by the second masking layer whereas the first masking layer left this area unprotected. Of course, we may apply the second masking layer without removing the first masking layer (or without removing it completely). The second masking layer may be applied over the hole/duct surfaces.
The second masking layer may be patterned (may be using standard lithographic techniques) and may be subsequently etched.
The second masking layer may be left in the shape of a disc, or a rectangle. In perhaps the preferred embodiment the second masking layer is left with a square remaining. The second masking layer may have its edges aligned with specific crystal planes of the first material. The second masking layer may be silicon dioxide.
In one embodiment the first material (which may be a bulk silicon crystal) may be etched in an anisotropic etch which undercuts the second masking layer. The crystal planes of the first material may have been arranged so that planes having a low etch rate bound the etching process creating the desired micro-projection structure. The second masking layer may then be removed. Such a process results in the micro-projection being formed from the first material.
In an alternative embodiment once the second masking layer has been formed so that it covers the inside surface of the duct, and in this embodiment it also covers the upper surface of the first material, the portion of the masking layer covering the upper surface of the first material may be removed so as to leave the second masking layer covering the inside surface of the duct.
The first material may then be removed from around the masking layer on the inside surface of the duct leaving the masking layer substantially intact. This may result in the micro-projection being formed from the material of the second masking layer (a second material). This may be thought of as fabricating a micro-projection from a second material which is different from that of the first material. The second masking layer may be thought of as the second material.
The second material may substantially completely fill the duct.
In such a case the method may comprise creating a duct in the first material, and lining the duct with the second material. Further steps in the method may be to subsequently remove the first material from around the lining of the second material. Such a method may leave the second material substantially remaining. It may be possible to have the first and second materials the same, if a suitable materials removal technique can be employed, but we prefer to have them different so as to allow their different properties to influence the removal rate of their materials.
In a variation of the method, the second material forms a lining of the duct or hole and a portion of the second material may be removed from the lining of the duct before, or whilst the first material is removed from around the lining. This allows the shape of the resultant micro-projection to be controlled.
The cross section of the micro-projection resulting from such a method may be varied according to the cross section of the duct which was created in the first material.
For instance the cross section may be circular, square, rectangular, elliptical, etc. Each of these cross-sections may be suitable for different applications.
The second material may be SiO2, which is readily formed around a duct should the first material be silicon. The SiO2 layer may be grown by oxidation of the silicon wafer or may be deposited (perhaps by Plasma Enhanced Chemical Vapour Deposition PECVD). Should the second material be a material other than silicon dioxide then the skilled person will appreciate that the second material would need to be deposited (again perhaps by PECVD).
The second material may be metal, or may be another conductor, or may be a ceramic, or may be a polymer. An advantage of using a metal, or a conductor for the second material is that an electrical conductor is produced which would allow currents and potentials to be applied to the micro-projection.
In another embodiment the method may comprise fabricating the micro-projection from the material and subsequently forming the duct through the micro-projection.
To achieve the formation of the micro-projection onto the surface of the first material a masking layer may be created on to the surface of the first material. The masking layer may be in the shape of a disc, or a rectangle. In perhaps the preferred embodiment the masking layer is fabricated as a square. The masking layer may have its edges aligned with specific crystal planes of the first material. The masking layer may be silicon dioxide.
Once the masking layer has been created the first material may be removed from underneath the masking layer by etching using an anisotropic wet etch. In effect, this etch undercuts the masking layer.
The crystal planes of the first material may have been arranged so that planes having a low etch rate bound the etching process creating the desired micro-projection structure. In such a process the anisotropic etch undercuts the masking layer until the slow etch rate crystal planes are met.
Preferably, the duct is fabricated such that the duct is coincident with an apex of the micro-projection.
The micro-projection may be formed by a number of techniques. For example any one of the following list may be suitable: anisotropic wet etching of silicon using liquid alkaline etches, focused ion beam milling, or transferring the pattern to the silicon from a domed resist mask using some form of plasma/ion beam etching.
The method may then comprise covering the micro-projection in a planarising layer. Next, the planarising layer may be etched using a standard lithography to reveal the tip portion of the micro-projection.
Once the tip portion of the micro-projection has been revealed the duct can be etched perhaps by using one of the following techniques: deep dry etching wherein the planarising layer may act as a mask; anodisation of the structure in HF acid; laser ablation; focused ion beam milling. Should anodisation be used it is possible (but it is uncertain) that a coarse tree like pore structure may be produced within the micro-projection. Further, anodisation may be of special benefit for forming small holes with a high aspect ratio. For example holes as small as 1 xcexcm may be made with an aspect ratio greater than 100 (this is explained in an article by V. Lehmann xe2x80x9cThe physics of macropore formation in low doped n-type siliconxe2x80x9d, Journal of Electrochemical Society Vol. 140. October 1993 pages 2836-2843). This is incorporated by reference and the skilled person is directed to read this article.
Once the method has been used to create the micro-projection and duct the method may further include linking the duct to a reservoir to create a device.
The first step in such a process may be to ensure that the duct passes entirely through the material being processed, ensuring that the remaining steps are easier to perform.
One possible way of doing this is to ensure that the duct is long enough to pass through the material. Alternatively, or additionally, matter may be removed from a side opposite a side of the first material where the micro-projection has been fabricated. Both of these techniques ensure that the duct passes completely through the material.
Next, the material may be attached to a piece of a second material. The second piece of material may have a channel which when the two pieces of material are attached connects to the duct and may link the duct to a reservoir.
The channel may be cut into an upper surface of the second piece of material, so providing a channel which may be easy to fabricate.
Alternatively, or additionally, a channel may be cut into a rear face of the first material.
The first and second materials may be the same material. Preferably, the two materials are silicon, preferably a hulk silicon single crystal. Alternatively, the first or the second material may be glass.
It is possible to bond glass to silicon using anodic bonding and it is also possible to etch channels in to glass.
In a further step of the method the surface of the materials (perhaps the first material, or perhaps the second material) may be modified. In one particular embodiment the surface of the first material may be porosified, so modifying the material""s interaction with biological systems. This is explained in an article by L. T. Canhamxe2x80x94xe2x80x9cBioactive silicon structure fabrication through nanoetching techniquesxe2x80x9d: Advanced Materials volume 7 page 1033 (1995). This is incorporated by reference and the skilled person is directed to read this article.
Such porosification may be performed by electrochemical anodisations processes (may be in HF acid) or possibly by immersing the structure into a stain etching solution.
The stain etching solution may be a mixture of hydrofluoric acid and nitric acid. This mixture is especially suitable for etching silicon.
The shape of the micro-projection may be altered by subsequent processing. For instance part of the micro-projection may be cut away. This cutting may be performed using ion beam etching or other similar processes.
Preferably the micro-projection has an axis which is substantially normal to the surface of the first material. Alternatively, an axis of the micro-projection may be inclined at an angle to the surface of the first material.
According to a second aspect of the invention there is provided a micro-projection having a base portion which is provided on the surface of a first material, and a remote or tip portion, wherein the micro-projection has a duct in at least a region of the tip portion.
The duct may pass between the tip portion and the base portion. Indeed, the duct may pass entirely through the micro-projection.
Alternatively, the duct may be provided as an indentation in a region of the tip portion of the micro-projection. This may be advantageous in that it would allow matter, perhaps a liquid, to be held in the indentation.
The micro-projection may be a micro-needle or may be a micro-barb. Alternatively, the micro-projection may be thought of as a micro-tube.
The micro-tube/micro-projection may be fabricated in a sheet of silicon or other semiconductor material, or indeed other crystal lattice material.
The base portion has a width of less than approximately 1 mm and preferably less than approximately 100 xcexcm, and may be less than approximately 10 xcexcm.
Preferably the micro-projection is transverse to the surface of the first material, and most preferably normal to it. This may make the micro-projection easier to fabricate and involve less processing steps than prior art techniques. Further, fabricating a micro-projection normal to the surface may allow an array of projections to be formed. It may be easier to connect the micro-barb to a channel. The micro-projection may of course be fabricated at an angle to the surface of the first material.
The micro-projection may be fabricated from the first material. This may make the micro-projection mechanically stronger than if the micro-projection were fabricated from another material.
There may be an array of micro-projections extending away from a common carrier, preferably extending perpendicular to the plane of a substrate sheet. The micro-projections may share a common reservoir. The common carrier may be the first material.
The micro-projections may have a length of up to substantially 2 mm. Alternatively, the micro-projection may have a length of up to substantially up to 500 xcexcm. In one embodiment the micro-projection may have a length substantially in the range 0 to 300 xcexcm.
The base portion of the micro-projections may be up to substantially 1 mm in width. Alternatively the base portion of the micro-projection may have a width of substantially 50 xcexcm or may have a width of substantially 10 xcexcm or may be 5 xcexcm.
The skilled person will appreciate that the dimensions of the micro-projection may be limited by the technique used to form the projection. That is the available aspect ratio of the fabrication technique may be the limiting factor.
A high aspect ratio dry etching process may be used to form the duct. Alternatively, or additionally, the duct of the micro-projection may be formed by the fabrication of porous silicon.
Should the method discussed hereinbefore, wherein a second material is used to line a duct, or hole, in a first material followed by subsequent removal of the first material, the duct may have dimensions substantially in the range of 200 xcexcm length, may be with a width of substantially 10 xcexcm.
There may be up to substantially 2500 micro-projections provided on a 5 mm square piece of material. Of course, the skilled person will appreciate that the number of micro-projections which it is possible to provide within a 5 mm square of material depends on the size of the micro-projections. The number quoted is a guide to the number which may be achieved and represents a realistic number.
The skilled person will realise that the dimensions of the micro-projections can be altered to suit the application by altering the fabrication parameters used to create the micro-projections.
The method may comprise using an array of micro-projections with the duct of each micro-projection being connected to a single reservoir.
In one embodiment the surface of the micro-projections may be porosified. An advantage of this is that the bio-activity of the surface may be controlled; especially should the first material be silicon. This is advantageous in some applications, for instance should the micro-projections be used in transdermal drug delivery. (By bio-active it is meant that the material induces a specific biological process. For example, it may induce calcium phosphate precipitation within living systems).
The needle may become bio-compatible because of the porosification. This is advantageous because if the needle breaks when being used to pierce a user""s skin it may be resorbed by the body reducing problems associated with the broken needle.
According to a third aspect of the invention, there is provided a micro-tube fabricated in the surface of a first material.
Preferably the micro-tube has a base portion with a width of less than approximately lmm and more preferably less than approximately 100 xcexcm and possibly below 10 xcexcm.
The micro-tube may be fabricated from a substance other than that of the first material. In one embodiment the first material may be silicon and the micro-tube may be fabricated from SiO2. Of course, other possible combinations of materials are possible. For instance metals, ceramics, or polymers may be used from which to form the micro-tube.
The micro-tube may have a cross-section that is substantially circular, possibly substantially square, possibly elliptical, possibly rectangular.
The micro-tube may comprise a micro-cuvette.
The micro-tube may be filled with a filler material. The filler material may be a different material to wall portions of the micro-tube or alternatively, the filler material may be the same material as the wall portions of the micro-tube. Indeed, the micro-tube may be thought of as a solid micro-projection, or micro-rod.
According to a fourth aspect of the invention there is provided a micro-analysis system in which a micro-tube according to the third aspect of the invention is provided and in which an analysis means is provided to analyse a substance within the micro-tube.
Such a system could prove beneficial in that it would allow samples to be analysed in a confined spaces. A fabricated micro-tube may be transparent to IR, UV and possibly visible light and, as such, would allow xe2x80x9con chipxe2x80x9d spectroscopy. That is spectroscopy within a system provided on a single chip or package.
The micro-analysis system could comprise more than one micro-tube and may possibly comprise an array of micro-tubes. Such an arrangement would allow more samples to be tested or allow a variety of samples to be tested at the same time. The micro-tubes may comprise micro-cuvettes.
Providing an array of micro-tubes is advantageous in that it would allow many samples to be tested at the same time.
The micro-analysis system may be combined with a micro-tube array acting as a micro-pipette array.
The micro-analysis system may comprise a delivery system so allowing chemicals, or other substance to be tested, to be inserted into the micro-tube.
The delivery system may comprise a micro-pipette or may comprise a duct fabricated into the first material, or may comprise an array of micro-pipettes or ducts.
The analysis means may comprise a light source, adapted to shine light through the micro-tube and a detector to detect light passing through the micro-tube or to detect the fluorescence generated by the incident light. This may provide a suitable way of analysing the contents of the micro-tube.
The light source may be an LED or may be a LASER. The light source (or the detector) may further comprise filters and may have beam shapers, lenses, etc. to ensure light having the desired properties passes through the micro-tube.
Such optical components may be fabricated using the same wafer of first material from which the micro-tube is fabricated. This is particularly advantageous should the micro-analysis system be reusable. This would make the process of fabricating the single wafer device more cost effective.
According to a fifth aspect of the invention there is provided a micro-manipulation system using the structure of the third aspect of the invention wherein the micro-tube is connected to a pump.
The pump may be a vacuum pump.
Such an apparatus may prove beneficial in that it would allow small objects to be picked up by the micro-tube.
The micro-tube may be connected to the pump via a duct in the material. This provides a simple structure.
Control electronics may be provided on the first material which controls how the device operates. Such a structure may lead to an integrated package, containing both the control electronics and the micro-manipulation system.
Alternatively, or additionally, microfluidic pumps, and/or micro-fluidic valves may also be provided on the first material. Such devices are advantageous because they allow the fluids within the micro-manipulation system to be pumped to different points as required. The micro-fluidic pumps may comprise a heating element. The heating element may be a resistor.
According to a sixth aspect of the invention there is provided a method of coupling an optical fibre to the first material comprising providing a micro-tube according to the third aspect of the invention and using the micro-tube as an optical coupling component.
Preferably the optical coupling comprises a wave guide. The wave guide may be a micro-tube or may be a micro-rod.
Optical may be intended to cover any electromagnetic wave, for instance radio, visible light, ultra violet, infra-red, etc.
Such a structure may provide a simple way of coupling the optical fibre to the material. It has the advantage over conventional techniques in that the fibre may be connected normal to the material as opposed to having to be orthogonal to the material.
According to a seventh aspect of the invention there is provided an optical coupling system adapted to couple an optical fibre to a substrate, the coupling system comprising a micro-tube fabricated on the surface of the substrate.
The optical fibre may be inserted into the micro-tube to achieve efficient coupling.
According to an eighth aspect of the invention there is provided a method of optically connecting two wafers of material together comprising providing on a first of the wafers a micro-tube according to the third aspect of the invention and coupling this to the second wafer in such a way so as to enable light to be passed between the two wafers.
This may allow signals, which may be optical signals, to be passed between the two wafers with low losses.
The method may comprise ensuring that light being transmitted passes entirely through wave guides (i.e. does not travel through free space). This has the advantage, over free space interconnection schemes, that dust and other such debris does not degrade the quality of the signal.
According to a ninth aspect of the invention there is provided a structure for optically connecting two wafers of material, the structure comprising a micro-tube fabricated on to the first wafer and being connected to the second wafer.
According to a tenth aspect of the invention there is provided an array of micro-projections.
There may be provided an array of micro-needles. Indeed, there may be provided an array of micro-tubes.
According to a eleventh aspect of the invention there is provided a micro-barb on the surface of a material having a base portion adjacent the first material in which the barb has a porosified surface.
Preferably the base portion has a width of less than approximately 1000 xcexcm and most preferably less than approximately 100 xcexcm and possibly below 10 xcexcm.
The barb may be formed by the processes described in relation to the first aspect of the invention.
The porosification may be performed by electrochemical anodisations processes (possibly in HF acid or possibly by immersing the structure into a stain etching solution). Possibly the stain etching solution is a mixture of hydrofluoric acid and nitric acid.